The present invention relates to a technique for amplifying the output signal of a sensor.
In recent years, the use of various kinds of sensors, such as a geomagnetic sensor and an acceleration sensor, is spreading rapidly. For example, these kinds of sensors are incorporated in a smartphone or the like to provide various services by using the direction and acceleration detected by the sensors. Since the output signals of these kinds of sensors are weak, the signals are generally used after amplified by an amplifier (refer to JP-A-11-144187).
In the case that the output signal of a sensor is amplified by an amplifier, it is frequent that the amplifier is operated in a fixed frequency band in which the amplifier can perform amplification (in other words, the amplifier is operated in a wide bandwidth in the case that high-speed operation is performed). The reason for this is described below. FIG. 7(a) is a graph showing the frequency characteristics of an amplifier in the case that the amplifier is operated in a narrow bandwidth and in the case that the amplifier is operated in a wide bandwidth (four times wider than the narrow bandwidth). FIG. 7(b) is a graph showing the settling time of the amplifier in each case of the narrow bandwidth and the wide bandwidth. The settling time is a time required until the output of the amplifier converges after a rectangular step input is applied to the amplifier (in other words, a time required until the output signal of the amplifier stabilizes). As shown in FIG. 7(b), the settling time is shorter in the case of the wide bandwidth. In the case that a physical state is detected by a sensor and an apparatus is controlled using the output signal of the sensor, in order that the change in the state is detected with a short delay and reflected to the control, it is necessary to operate an amplifier for amplifying the output signal of the sensor in the wide bandwidth and thereby to shorten the settling time of the amplifier. However, in the case that the amplifier is operated in the wide bandwidth, noise is also amplified and the S/N ratio of the signal after the amplification is lower than in the case that the amplifier is operated in the narrow bandwidth, whereby there occurs a problem that small changes in the output signal of the sensor cannot be captured.